Heart disease is a major cause of deaths in the United States and in other industrialized nations. One well-known treatment approach utilizes an implantable cardiac pacing device, through which relatively mild periodic electrical impulses are applied to epicardial or endocardial tissue as necessary to maintain normal sinus rhythm. More recently, cardioversion/defibrillation devices have been developed to counteract tachyarrhythmias (rapid disturbances in cardiac electrical activity). In particular, the conditions of ventricular tachycardia, ventricular flutter and ventricular fibrillation are widely believed to be the primary cause of sudden deaths associated with heart disease. Defibrillation devices also are utilized to counteract atrial tachyarrhythmic conditions, although such conditions are not considered life threatening unless they lead to a rapid ventricular disturbance or unless persistent enough to cause blood clots to form within the atrium because of pooling caused by poor atrial ejection.
Tachyarrhythmic conditions frequently can be corrected by applying relatively high energy electrical shocks to the heart, a technique often referred to as cardioversion. Cardioversion devices include implantable electronic standby defibrillators which, in response to the detection of an abnormally rapid cardiac rhythm, discharge sufficient energy through electrodes connected to the heart to depolarize and restore the heart to normal cardiac rhythm.
Cardioversion/defibrillation devices frequently include epicardially implanted electrodes. The surgical procedure required for implantation, i.e., thoracic surgery such as a median sternotomy or thoracotomy, is highly invasive and presents significant risks to the patient. It is highly desirable therefore to make every effort to reduce the number of times that invasive surgery is needed for a patient by having the life of each implanted device extended as long as possible. For this reason, extensive design and programming efforts are used to maximize the life of battery cells associated with cardiac pacing devices.
Electrodes implanted in the body for electrical cardioversion or defibrillation of the heart are well known. More specifically, electrodes implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life-threatening cardiac arrhythmias, where electrical energy is applied to the heart via the electrodes to return the heart to normal sinus rhythm. See, for example, U.S. Pat. No. 4,291,707 to Heilman, relating to a planar patch defibrillation electrode.
The Heilman patent specifically discloses an implantable cardiac electrode comprised of a planar conductive material insulated completely on one side and partially on its other side. Apertures are provided around the insulated perimeter of the partially insulated side of the electrode to provide for efficient and uniform energy transfer to the heart tissue by eliminating the so called "edge-effect".
The amount of energy delivered by the electrodes to the heart during defibrillation (or cardioversion) depends on the placement of the electrodes and the ability of the electrodes to distribute the energy uniformly throughout a major portion of the heart. This energy is called the defibrillation or cardioversion energy.
U.S. Pat. No. 5,312,442 relates to the art of implantable cardiac defibrillators, and in particular, is related to an energy dissipation resistor capable of efficiently and reliably dissipating energy stored in the capacitor(s) of an implantable cardiac defibrillator. It is often the desire to dissipate electrical energy stored in defibrillator capacitors, rather than discharging to the heart, by diverting the capacitor voltage to an internal resistor. This is commonly referred to as an "internal dump". See, for example, U.S. Pat. Nos. 4,316,472 and 4,488,555 to Imran and Mirowski, respectively, where internal load resistors are shown. Presently, conventional resistive elements, such as carbon or ceramic resistors, and the like are used to dissipate the energy. Such conventional resistive elements tend to be large and bully, and therefore difficult to package, often requiring extensive incoming inspection processes to assure that a desired reliability is achieved. In addition, although conventional resistive elements have proven generally effective in practice, worst case testing with multiple shock and internal dump episodes has resulted in heating of the resistive element and occasional destruction thereof.
For example, U.S. patent application Ser. No. 08/550,835, filed on Oct. 31, 1996, in the name of M. Brook and titled "Power Management System for an Implantable Device", shows a process for managing a power source, the power source having an output voltage, comprising the steps of:
periodically switching a load across the power source using a switch; PA1 monitoring the output voltage of the power source using a dedicated voltage monitoring device and current monitoring device; PA1 if the current monitoring device detects a current equal to or greater than a predetermined current threshold, opening the switch for one switching period; and PA1 if the output voltage is less than a selected threshold voltage, opening the switch until the output voltage is greater than the selected threshold voltage. This process is designed to and effectively does reduce the overall power utilization of a pacing system and can act to extend the life of a digitally charged power supply. PA1 a. monitoring a feature within the cardiac pacing device which indicates the relative strength of a battery compared to a defined initial battery strength, PA1 b. as said monitoring shows a decrease in said relative strength of the battery compared to said initial strength battery strength, selecting an alternate voltage multiplier setting by modifying the charge transfer capacitor configuration during the filling and output phase in said cardiac pacing device which are used to store charge in a power supply for a storage capacitor at a frequency of switching, and PA1 c. increasing the frequency of switching at which the at least two charge transfer capacitors are alternatively reconfigured between filling and output phases.
Cardiac pacing systems can tend to be quite active, with many different functions (sensing, pacing, charging, defibrillating, etc.) being provided by a single complex system. The activity of the system provides a significant drain on the usually single implanted battery which powers the entire system. Because the replacement of a battery is an invasive procedure which should be minimized in its frequency, anything which can be done to optimize the performance of an implanted pacing system or in anyway reduce the drain on the power supply to extend the life of the battery is a desirable goal.
Cardiac pacing systems are almost exclusively electronic, with no moving parts, so energy usage is already at relatively efficient rates. Any savings in energy usage is therefore very significant in extending the life of the battery in the pacing unit and in avoiding any invasive medical procedures. There are three general areas in which power utilization may be controlled: a) assuring that no work in performed when the inherent performance of the heart occurs, b) optimization of the function and power utilization of the components themselves, and c) programming of the pacing device to control the output of energy. Any new variation within these areas or new procedures which can be executed to reduce energy usage and prolong the life of the battery are highly desirable, in addition to the specific means shown in U.S. patent application Ser. No. 08/550,835 shown above.
Those skilled in the art will appreciate that the body has several mechanisms designed to adjust to metabolic demand by modifying cardiac output and, ultimately, oxygenation of tissue. The cardiac output may be increased or decreased by adjustment in sinus rate or stroke volume. Recognizing this fact, it has been possible to monitor metabolic demand by monitoring the sinus rate, the stroke volume and/or other activities which indicate different levels of use required by the patient's system. Both the monitoring of the patient's system and the exercise of these variations in pacing frequency require additional energy utilization and energy drain on the battery cell, and system must be put into effect to allow for these functions to be provided without shortening the life of the standard batteries used within the field.
U.S. Pat. No. 5,397,342 relates to body implantable tissue stimulation electrodes, e.g., for cardiac pacing or cardioversion/defibrillation, and more particularly to the deployment and implantation of such electrodes.
Examples of epicardial defibrillation electrodes are found in U.S. Pat. No. 4,567,900 (Moore), U.S. Pat. No. 4,291,707 (Heilman et al.), and U.S. Pat. No. 4,860,769 (Fogarty et al.). A pair of differently biased (e.g., oppositely polarized) epicardial electrodes can be employed, as shown in Moore. Alternatively, the Heilman patent discloses an intravenously inserted endocardial electrode arrangement in combination with a patch electrode positioned near the left ventricular apex.
U.S. Pat. No. 4,270,549 (Heilman) describes a technique for inserting and placing defibrillation electrodes, involving intravenous insertion of an endocardial electrode in combination with a patch electrode inserted through a skin incision and through a tunnel created inside the thorax and outside the pleural cavity. Alternatively, U.S. Pat. No. 4,865,037 (Chin et al.) discloses a technique for inserting separate electrodes into the intrapericardial space through catheters. An incision is formed in the upper abdominal wall. Then, tissues between the incision and pericardium are separated, and an incision is then made in the pericardium. A cannula containing a defibrillation electrode is inserted through these incisions, to enable positioning of the electrode in the pericardium. A second cannula containing a second electrode is inserted on the opposite side of the heart, in the same manner.
A problem, particularly with patch electrodes, is the current density gradient, i.e., maximum current density regions at the patch periphery. Current density gradients reduce the efficacy of the electrode, in terms of the ratio of useful cardioversion/defibrillation energy as compared to required pulse generator output energy. Alternative energy conserving means are desirable to overcome or balance these types of inefficiencies in cardiac pacing devices.
With the criticality of extending the life of battery cells and in maintaining a potentially large number of functions within the pacing device, any modification which can measurably extend the life of the battery can be advantageous to a cardiac pacing system.
To minimize the size of an implantable pacing device, the power source is usually a single battery cell for a brady pacemaker, and one or perhaps two cells of different chemistry types for an implantable defibrillator. Different battery types may be used within the same implantable defibrillator, one optimized for low-power high energy density for powering the internal circuitry of the device, and a separate high-power battery to provide the infrequent high energy charging of the defibrillator high output voltage capacitors.
The open circuit output voltage of these battery power sources are typically very low (e.g. 2.0.about.3.5V) because the batteries are often composed of a single cell. Therefore, fundamental to cardiac pacing devices and defibrillator design is the need for step-up voltage power supply circuitry. Defibrillators require the battery voltage to be stepped up to 100's of volts to charge the defibrillator output capacitors. Even brady-cardia pacing can require pace pulse amplitudes larger than can be delivered from the battery, the maximum programmable output amplitudes varies according to manufacture but is typically between 4.5 to 7.0V, because occasionally these higher pace amplitudes are required to properly depolarize the heart depending upon pacing lead type, placement, and health of the patient's heart.
There are many known circuit topologies for generating step-up as well as step-down voltages, as indicated by the representative reference list below:
K. Asano, "Voltage Dropping Circuit", U.S. Pat. No. 4,205,369, May 27, 1980. PA0 J. F. Dickson, "On-chip high -voltage generation in MNOS Ics using an improved voltage multiplier technique", IEEE J. of Solid-State Circuits, vol, SC-11, pp. 374-378, June 1976. PA0 R. Gregorian and G. C. Temes, "Analog MOS Ics", Wiley, 1986, pp 463-464. PA0 I. Harada et al., "Characteristics analysis of Fibonacci-type SC transformer", IEICE Trans. Fundamentals, vol. E75-A, no. 6, June 1992, pp 655-662. PA0 F. Krummenacher et al., "Higher sampling rates in SC circuits by on-chip clock voltage multiplication, Proc. ESSCIRC, pp. 123-6, September 1983. PA0 D. H. Oto et al., "High-voltage regulation and process considerations for high-density 5-V-only EEPROMs", IEEE J. of Solid-State Circuits, vol. SC-18, pp. 532-538, October 1983. PA0 J. C. Ryan, K. C. Carroll and B. D. Pless, "A four -chip implantable defibrillator/pacemaker chipset," Proc. IEEE 1989 Custom IC Conf., pp. 7.6.1-7.6.4, May 1989. PA0 S. Singer, "nductance-less up dc--dc converter", IEEE J. of Solid-State Circuits, vol. SC-17, pp. 778-781, August 1982. PA0 D. R. Squires, "Monolithic Voltage Divider", U.S. Pat. No. 4,433,282, Feb. 21, 1984. PA0 F. Suzuki and S. Ichikawa, "DC-to-DC Voltage Converter", U.S. Pat. No. 4,451,743, May 29, 1984. PA0 U. Tietze and Ch. Schenk, Electronic Circuits, Sringer-Verlog, Berlin, 1991. PA0 F. Ueno, T. Inone and T. Umeno, "Analysis and application of SC transforms by formulation", Electronics and Comm. In Japan, Pt. 2, vol. 73, no. 9, 1990, pp. 91-103. PA0 J. S. Witters et al., "Analysis and modeling of on chip high voltage generators circuits for use in EEPROM circuits," IEEE J. of Solid-State Circuits, vol. 23, pp. 1372-1380, October 1989. PA0 D. Wayne et al., "A single-chip hearing aid with 1V SC filters", Proc. IEEE 1992 CICC, pp. 7.5.1-7.5.4, May 1992.